Prior to the implementation of Voice over Internet Protocol or IP (VoIP), the basic methodology used in configuring private branch exchange (PBX) systems remained unchanged for many years. These methods remain valid for non-IP switching systems and, in this application, are documented in the original AT&T 555-series traffic manuals.
The primary focus of the older systems was on the call usage for stations (e.g., circuit-switched telephones) and trunks. A call is a switched, connection between two ports. Call usage is the total connection time for all calls for a given period of time. Call usage is traditionally partitioned into components (such as trunk-to-station (incoming), station-to-trunk (outgoing), station-to-station (intercom), and trunk-to-trunk (tandem) calls) using a call tree. Given this partition, traditional methods assume a balanced system and use tables to derive the traffic load on system components, such as the port network Time Division Multiplexed or TDM buses. A “balanced system” refers to a system in which the traffic load is uniformly distributed among all network switching components, such as TDM buses, while an “unbalanced system” refers to a system in which the traffic load is non-uniformly distributed among all switching components. For example, the impact of outgoing trunk calls on the TDM bus is calculated using a first port network preference link table that was originally derived by a Bell Labs mathematician. System components that are configured include port network time slots, trunk groups, tone resources, announcement resources, inter-port network links, and processor occupancy.
The inclusion of IP stations and trunks in a single system required a separate calculation for media processor and Control Local Area Network (C-LAN) resources. The TDM and other calculations remained the same. When shuffling and multiple network regions were introduced, the calculations were changed to include some workarounds. These methods were acceptable for small-scale deployment of IP stations and trunks. The advent of large scale, multiple network region deployment, IP center-stage connect, and new types of non-traditional port networks and gateways, however, require a complete rethinking of the configuration methods.
Converged PBX systems, in particular, have introduced multiple layers of complexity into the modeling of communication networks. Converged PBX voice systems carry traffic over both TDM and IP data facilities. As a result of the increasingly widespread use of converged PBX systems, single enterprise switching networks may now have stations, trunks, and processing gateways dispersed over several continents as a highly unbalanced system. Furthermore, calculations based on call usage have become overwhelmingly complex due to the statistically large number of call type combinations that are possible. The advent of IP has increased dramatically the number of types of calls for a balanced system let alone for an unbalanced system. For example, in an enterprise with n network regions, the number of call usages that would require estimation would increase n-fold.